Various types of equipment use high-frequency high voltage to establish an electric arc for transport of material and/or energy. The level of disturbance in such equipment is often extremely high. This to a very great extent makes it difficult to carry out measurements in the equipment and to control the said equipment. Examples of areas of application for such equipment are thermal spraying, cutting and welding. In this document, welding will be used as an illustrative example.
In arc welding it is often necessary to measure and to feed back the voltage across the arc to the control part and/or measurement part of the welding current source. In welding with consumable electrodes, the fed-back measured value is often used to regulate the welding process in order to obtain an even droplet transfer with minimum weld splatter. In welding with non-consumable electrodes, the aim of the feed-back is in most cases to prevent dipping of the electrode in the molten pool, which would lead both to a destroyed electrode tip and to inadmissible inclusions of electrode material in the weld deposit. A special case is mechanized TIG welding when joining pipes or welding pipes into end plates, where the arc length and thus the height of the welding electrode above the workpiece are regulated as a function of the arc voltage. This height control is used, for example, to bridge variable roundness of the pipes or to adapt the height in multiple pass welding, when the weld seam laid during the preceding pass is contacted. In welding both with consumable and non-consumable electrodes, the aim can also be to register welding data for the purpose of monitoring quality.
In order to create reliable arc ignitions when the quality and/or productivity requirements are high, a high-frequency device (HF device) is often used which superposes high-frequency high voltage on the open-circuit voltage fed from the current source to the welding electrode. An HF spark ionizes the protective gas between electrode and workpiece and the arc can be established. The HF device is then switched off normally, and the arc is maintained thereafter with ordinary welding voltage. When welding with alternating current, it may also happen that HF is not only used at the outset, but is also used in order to keep the arc burning at the zero passages of the welding voltage.
A transformer is often used to connect the HF voltage to the welding cable. A characteristic feature is that the secondary winding of the transformer is dimensioned to withstand the total welding current. This transformer transfer can be done for reasons relating to personal safety. When the HF device in question permits connection without transformer, there is often an inductor with or without core connected between current source and HF device in order to prevent the HF voltage from entering the power part of the current source.
On account of high welding currents and sometimes long welding cables, the voltage drop across the cables is considerable if the voltage measurement takes place at the welding current source connections. For this reason, the measurement advantageously takes place in direct proximity to the electrode, and a separate measurement line leads the voltage value back to the current source. Since the return line from workpiece to current source can often be dimensioned for minimum voltage drop, while the welding cable must be dimensioned smaller on account of requirements relating inter alia to ease of handling for the welder, or automatic welding torch manipulator, the measurement in these cases can take place relative to the return cable socket of the current source, and thus a single measurement line is sufficient.
As was mentioned in the introduction, a problem with the said HF voltage is that it causes considerable disturbance for the electronic components in the current source. Operating breakdowns and component malfunctions are the result, unless the electronics system is screened off with very great care.
It is known (as is shown diagrammatically in FIG. 1) to arrange a filter in the measurement line in order to prevent the HF voltage from reaching the control part of the current source.
A second problem at the high frequency is the difficulty involved in maintaining sufficient impedance with respect to the rest of the equipment so that the HF voltage is not damped en route from HF device to welding electrode.
In dimensioning the above filter, a compromise has to be reached between good damping of the HF voltage with respect to the control and/or measurement part, low damping of the HF voltage at the electrode, and low damping of the measured welding voltage.
A further problem is that the high HF voltage breaks down the insulation upon long-term exposure unless use is made of large insulating distances and high-quality insulation material.
Because of this latter problem, the above-mentioned filter must be made extremely robust. High-quality components, large insulating distances, requirements in respect of casting in order to avoid corona effects, etc., lead to large physical dimensions and high costs. In order to avoid capacitive damping of the HF voltage, the filter has to be placed near the electrode. The dimensions mean that the measurement line often has to be laid physically separate from the welding cable, which further adversely affects the HF voltage, as the damping via the capacitance to earth increases when these are laid on a floor, for example.